The chest x-ray is used for detecting a number of patient conditions and for imaging a range of skeletal and organ structures. Radiographic images of the chest are useful for detection of lung nodules and other features that indicate lung cancer and other pathologic structures and other life-threatening conditions. In clinical applications such as in the Intensive Care Unit (ICU), chest x-rays can have particular value for indicating pneumothorax as well as for showing tube/line positioning and other clinical conditions.
The chest region includes a wide range of tissues, ranging from rib and other bone structures to the lung parenchyma. This greatly complicates the task of radiographic imaging for the chest region, since the different types of bone and tissue materials differ widely in density. Optimization techniques that have been developed for chest imaging employ making a number of compromises to provide a suitable signal-to-noise ratio (SNR) and sufficient contrast-to-noise ratio (CNR) for soft tissue. In spite of these image processing techniques, the broad range of densities and structures leads to what has been termed “anatomical clutter”, making it particularly challenging to interpret chest x-ray images in many cases.
Chest radiographs are often used to examine the lung parenchyma, for which tissue/air contrast is an important feature. As indicated in published work either based on Monte Carlo simulations (such as, for example, in the article entitled “A comparison of mono- and poly-energetic x-ray beam performance for radiographic and fluoroscopic imaging,” J. M. Booth et al, Medical Physics, vol 21, no. 12, 1994) or based on experimental measurements (such as in “Investigation of optimum X-ray beam tube voltage and filtration for chest radiography with a computed radiography system,”, C. S. Moore, The British Journal of Radiology, vol. 81, 2008), the optimal kV range for soft-tissue and air contrast, for average-sized adult patients, is from 60 to 80 kVp using poly-energetic x-ray beams at the same effective patient dose. However, this is rarely used in practice. Instead, the x-ray exposure technique that is routinely used for in-room posterior-anterior (PA) view chest radiography specifies 110 kVp to 130 kVp. This higher kVp range is used because it helps to reduce bone contrast. In chest images, the bone contrast from the surrounding rib cage is reduced in order to allow better visibility of the underlying tissue. The Monte Carlo simulation described by Booth et al. indicates that, with increasing exposure kVp, bone contrast decreases at a faster rate than soft tissue contrast decreases. Therefore, acquiring chest images at higher kVp helps to mitigate the bone contrast while still maintaining a reasonable level of soft tissue contrast. However, as a result of this compromise, the contrast of the lung parenchyma is less than optimal. This complicates the job of diagnosis and makes it more likely that image features be misinterpreted.
Another aspect of using higher kVp levels for chest imaging relates to increased x-ray scatter. Scatter reduces image detail contrast and increases noise levels, both of which hinder diagnostic accuracy. X-ray anti-scatter grids are frequently used to reduce scattering, but have negative effects as well. Grids of higher ratios are employed at higher energy levels, increasing the amount of incident exposure that is needed to compensate the exposure loss, but at the expense of increased patient absorbed dose.
A further issue relates to the need for imaging both bone and soft tissue in some patients. Booth et al indicate that 50 kVp is an optimal setting for bone contrast. Since standard chest exams are performed at higher kVp, typically around 120 kVp as noted earlier, rib bone contrast is greatly reduced in the images obtained, with correspondingly reduced bone detail conspicuity for diagnosis. Thus, patients for whom both thoracic bones and lung regions are examined often undergo two separate examinations, one radiograph taken at the 120 kVp level, another taken at 70 kVp. Because multiple views may be desired, a patient may need to undergo more than two exposures for a chest exam, one set of exposures optimized for lung fields, the other optimized for thoracic bones. Thus, the need to image at two different kVp levels can directly translate to double or even triple the exposure dose to the patient.
The use of lower energy x-ray photons helps to maximize soft-tissue and bone contrast in chest radiographs, but there can be negative effects if not applied appropriately. Lower energy photons become absorbed quickly by human tissues as the poly-energetic x-ray beam penetrates the patient. The negative impact of absorption is two-fold: 1) potentially increased absorbed dose to the patient, and 2) “beam hardening” effects. Beam hardening essentially modifies the x-ray spectrum at different positions in the image and reduces the effectiveness of radiation that is otherwise optimized for chest imaging. This effect becomes worse as patient size increases.
Dual energy (DE) imaging has been used as an alternative method for reducing the anatomical clutter that is typical of the chest x-ray. In DE imaging, low and high kVp exposures follow each other in close succession, so that their results can readily be combined without extensive registration techniques. No segmentation of rib features is needed; a weighted subtraction, pixel-by-pixel, can provide a degree of rib suppression that yields an image that can be more accurately interpreted.
There are some aspects to conventional DE imaging and resulting recombination techniques, however. One aspect of DE imaging relates to high dose levels, often as much as 1.5 to 2.5 times higher than standard chest x-ray exposures. Motion artifacts often result, leading to misregistration due to breathing, heartbeat, and other movement of the patient.
Even where motion artifacts are minimal, however, overall image quality is compromised in DE image processing. In dual energy imaging, the soft-tissue contrast-to-noise ratio (CNR) in the clear lung region (e.g., regions without any overlaying bone structures) is compromised in the soft tissue image during the subtraction operation between the high and low energy images. Since the clear lung region has no bone structures, it is desirable to maintain the CNR of the originally captured data.
Another technique that has been used for chest x-ray imaging is rib suppression. Rib suppression processing has been used to reduce rib effects without introducing additional radiation dose by operating on a standard chest x-ray image in regions where there are overlapping bone structures. However, the performance of rib suppression processing depends on the segmentation accuracy of the bone analysis algorithm. Because the image analysis is performed on the standard chest image, and soft-tissue and bone structures overlay each other, it can be very difficult to separate the bones from the underlying soft tissue structure. In some situations, one or more rib bones, or sections of rib bones, are missing from the segmentation analysis and are, therefore, not accurately suppressed. In some situations, some of the soft tissue structures may be mistakenly identified as bone structures, therefore being negatively suppressed.
Thus it can be seen that even though various approaches have been used to mitigate anatomical clutter in chest x-ray images, each of these approaches have their limitations. There is a need for solutions that provide the practitioner with chest x-ray images with reduced dose to the patient while at the same time exhibiting good contrast across the whole image.